Acoustical study and correction of auditoriums



Apr-il 15, i930. R, F NRRIS 1,754,332

ACOUSTICAL STUDY AND CORRECTION OF AUDITORIUMS Fild April 13. 1927 ggf vPatented pr. l5, 1930 UNITED STATES PATENT OFFICE RALPH FORBUSH NORRIS, OF MADISON, WISCONSIN, ASSIG-NOR T0 C. F. BURG-ESS LAB- ORATORIES, INC., OF MADISON, WISCONSIN, .A CORPORATION OF DELAWARE ACOUSTICAL STUDY ANI)` CORRECTION OF AUDITORIUIVIS Application filed April 13,

My invention relates generally to the acoustical study and correction of rooms, such as theaters, churches, or other auditoriums, and

'has particular reference to the controll or elimination of echoes in such rooms.

One of the objects of my invention is to provide a means for showing and determining which surfaces of a room, either proposed or existing, cause echoes.

Another object of my invention is to provide a means for showing'and determining the direction or directions in which sound is traveling at predetermined given Apoints in rooms, such as theaters, churches, or other auditoriums,

Another object is to vmake a record, as by photography, of the sound paths and sound concentrations in rooms.

Sound waves consist of a traveling succession of relatively high and low pressure areas. These areas move forward in air at approximately 1088 feet per second at ordinary room temperatures. lVhen these waves strike a comparatively smooth and non-porous surface, they are reflected from it. As

with light waves, if the sound wavev strikes a reflecting surface 'in a direction normal to its plane', it is reflected back toward its source. If the wave strikes the surface at an angle, it is reflected from the surface at the same angle. After a soundwave has been generated in a room, it is reflected from surface to surface, producing an enormously intricate sound pattern in the room. The laws of sound reflection are like those of light, and every surface' that the sound strikes may be considered, in some respects, as a secondary source of sound. As a mirror absorbs some of the light'that lfalls upon it, so surfaces struck by sound waves absorb a portion of the energy in the waves, the percentage being dependent upon the nature of the surface. The result is that the reflected sound in a room becomes con-V tinually weaker as its energy is partially absorbed by the successive reflecting surfaces. The length of time that it takes for a sound of standard intensity to be so reduced by absorption thatV it becomes inaudible, is called the period of reverberation of the room.

An echo is closely related to reverberation.

1927. serial No. 1s3,343.

The human ear is so constituted that it canl whose lengths differ by one-iifteenth of 1088 feet, or approximately 7 2 feet, the Sound will reach the ear of the auditor as two distinct sounds 1/15 of a second apart. When this occurs, thesound Wave reaching the ear latest is called an echo. When the diiference in the length of the two paths is more than 7 2 feet, the echo will follow the direct sound by a time interval even greater than 1/15 of a second. lVhen one or more echoes are produced in a room, a confusion of Sounds I'` sults and the room isv described as having poor acoustics.77 My invention 1s d1rected to means for determining those'surfaces which produce confusion echoes.7

The usual method now employed for finding echo-producing surfaces in rooms l1s graphical. In using this method it is necessary to construct scale drawings of cross-sections of the room, to lay out rays of sound directly from the source of sound, and indirectly as reflected from smooth nonabsorbent surfaces, making the angle of reflection of the sound ray equal to the angle of incidence. In this way, a series of lines may be built up which will show the sound paths and areas of sound concentration. Echoes may be detected by determining the difference in the lengths of the respective paths by which sound will travel from the source to any given point in the room, anA echobein produced when the `difference in the paths 1s, rou hly, 70 feet or more. This method is very la orious and less accurate than my improved method.

The present invention dispenses with the graphical effects and substitutes visual effeets which may be produced instantaneously and varied at will, The new method providing visual effects is capable of retaining all the advantages ofthe graphical method upon photographing the visual etiect.

In carrying out my invention, I utilize the similarity as to reflection between sound and light Waves. However, the high speed with which light travels (187,000 miles Der second) and the ease with which lig it rays may be photographed, as, for example, by

' allowing them to act on sensitizedpaper, or

graphs, the changing contour of a soung Wave may be shown as it strikes obstructions, such as surfaces, or as it interferes with other sound waves, or where it is reflected by surfaces. During the period that the wave front is relatively simple in formtliis method may give satisfactory results, but when the sound wave is broken up by striking various obstructions it becomes difficult to obtain satisfactory results (see Acoustics in auditorium design by P. E. Sabine in American Architect, June 18, 1924) In this method a small model of a room is erected and successive photographs of a single sound Wave emanating from the designated source of sound (such as the stage in a theater) are made. This method is very cumbersome and requires a highly developed technique. It has the further disadvantage that it shows the progress of a single wave only, whereas sound is composed of a rapid succession of waves which will often create acoustic conditions differing Widely from the pictures of the single wave.

In my improved method, I replace the sound Waves emanating from the designated sound source by light rays. I am often able to obtain complete information about the echo-producing surfaces of a room or auditorium in one picture or photograph, and nearly always in three or four. locate the source of the loudest echoes, but I also locate the source of secondary echoes, and I am also able to show visually the direction and intensity of sound at any predetermined point in a room. My method instantaneously shows the sound pattern produced by a multiplicity of sound waves rather than that produced by a single wave front, and therefore shows the normal condition in a poom, with the utmost simplicity.

The claims of this application are directed to the apparatus for attaining the viesults enumerated. In the divisional application No. 332,118, filed January 12, 1929, which issued August 27, 1929, as Patent No. 1,726,-

499 I have claimed themethod broadly and I not only ,method claims directed to another species.

In the accompanying drawings.-

Figure 1 illustrates a photo-picture made by my process which shows the acoustics of a room, such as a theater auditorium having echoes.

Fig. 2 illustrates a photo-picture of the improved acoustics of the theater of Fig. 1, after being corrected acoustically.

Fig. 3 shows one form of apparatus used in carrying out my invention. l

By way of illustration in the preferred manner of carrying out my invention, I firstinake contours of aroom, utilizing both plan views and various horizontal or vertical cross-sections, depending upon thepa'rticular problem Linder consideration. These miniature contours may preferably be from 12 to 2l inches long, but must be faithful reproductions as to the form of the original. When the contour has been reproduced on paper or cardboard, or in other convenient form, a strip of polished metal or other reflecting material, about one to two inches Wide, is j erected on edge along the plane contour of the room, providing a fence-like contour'that may be considered as a slice-section of the auditorium. The fence may be a single strip,

or a number of sections. In Fig. 3, --the char- Accordingly, in Fig. 3, the strip 6 is not lightreflecting. It therefore becomes necessary to ,know the types of surfaces in the room in order to duplicate exactly the conditions in miniature.` Surfaces vary in their ability to absorb sound, as is Well known to those skilled in the art. Since continuous smooth surfaces are good reflectors ofsound and are potential sources of echoes, I simulate these surfaces inthe miniature contour by leaving the corresponding areas of the reflecting strip polished. The areas which are sound-absorbing are treated so that they do not reflect light, or these sections of the contour strip may be entirely omitted. Occasionally, it only becomes necessary to erect the contour, of the ceiling in miniature, since the ceihng is sometimes the only portion havin(r sound-reflecting areas. .Side walls may also be sources of echoes. If the room under consideration is the auditorium of a theater, as shown in Fig. 3, there may be a balcony 7, which must be inserted for reasons which will'become apparent. The source of sound is considered to be on the stage 8. A small light 9, preferably an incandescent bulb, is` placed at this point.I

In one application of my invention, after the miniature contour 'fence is completed, it may be slipped over a sensitized surface, such as a sheet of photographic paper, the strips of polished metal or other reflecting or treated surface forming the contour being at right angles to the sensitized surface. This operation is of necessity carried out in a dark room, or in properly subdued light. The light 9, if an incandescent bulb, should have a concentrated filament and should be placed so as to illuminate both the contour strip and the entire sensitized surface which c'orresponds to the area traversed by the sound. This light, being connected to a suitable source 0f electrical energy, is Athen flashed. The intensity or length of liash should be such that any concentrations of light which are l caused by the curvature or angle of the metal wall will stand out in contrast to the balance of the sensitized surface. These concentrations oflight closelyapproximate the concentrations of sound in the auditorium. The exposed sensitized surface may then be developed or otherwise treated to fix the image permanently, as in aphoto-print.

In anotherfmodification of my process, instead of placing the contour strip on a sensitized surface, I erect it on any uniformly colored plane surface.' The light at the point of sound emanation is turned on. The plane surface is illuminated in Varying intensity, in proportion to the concentration of sound. This allows the acoustics of the room to be studied, so that'the observer may treat the reflecting surfaces, or carry out other ideas and observe immediately the effect of such treatments. Instead of replacing the plane surface with a sensitized surface, as hereinbefore described, the plane surface may be photographed directly by any suitable apparatus in the usual way. Any number of photo-prints may then bel made from the negative obtained in this way, the areas of light concentration being light instead of dark, as in the process where the contour is mounted on the sensitized surface.

When it is desired to know in what direction sound is traveling at definite points, as 10 to 29, throughout the room or auditorium, pins, needles, pegs, or similar articles, hereinafter called pins, may be inserted at these. points of the surface on which the contour is erected. It is especially desirable to know the source of sound at the points where the audience is located; `The pins cast shadows, as

4shown in Figs. l and 2 by dotted vlines of which the spacing increases as the shadow fades out. The intense end of a shadow pointsin the direction of the light source, either di` rect or reflected. sIn this way, it is often found that at 'some points in a room, as in. Fig. l, the audience hears the sound-from as sound pattern obtained for a theater having poor acoustics. In the miniature, or slice model, the light was placed at 9 on the stage 8. The print shows that the ceiling curvature at 1 causes echoes in the audience at 12, 13, 14, l5, 16 and 17. The pins inserted at these points show that the sound will be heard principally from two points, the stage 8 and the curved surface 1. Since the distance from the stage to the curved surface and back to points 12 to 17 is more than 72 feet greater than the direct distance fromthe stage, the curved surface produces bad echoes for the audience, especially in the region from 12 to 17 The curved surfaceV 2 also causes some echo at 16, 17 and 18, making the acoustics very poor 'at 16 and 17. l At 18 sound is also heard refiected from section 3 of curved surfaceA 3. Section 8 also reflects the sound into lthe rear of the balcony, at 22, thereby re-inof an auditorium, it is customary to take one picture of the reflection of the side Walls on the' Hoor plan; another picture of the reflection through the longitudinal section of the 1 l room; and a third picture on the transverse section of the roomf In athis way all of theV concentrations which cause echoes can be detected, together with'the surfaces from which they emanate. In the case of new buildings, the angles and curvatures of the surface mayl be changed to eliminate these echoes, but 1n the case of buildings already constructed, sound-absorbing material may be placed on these surfaces in such a way as to eliminate the echoing effect.

Various departures .from and modifications of the method and apparatus herein described are, of course, contemplatedt -In the foregoing example, the reflection of hght rays proper have been employed to indicate the pin 24ycasts a shadow 24 which is reflected forwardly as a shadow Qllbby the rear wall 5. Thus, a beamof light or a shadow may be employed and its reflection studied.

What I claim as new is:

1. An lapparatus for showing the paths and intensity'of sound waves from Ia given source in a room, comprising a plane surface on which is mounted a miniature fence-like member having the contour of a cross-section of said room, in combination with al source of light positioned at a point corresponding to the point of sound cnianation in said room.

' 2. An apparatus for showing the paths and intensity Iof echo-producing surfaces in a room, comprising, in combination a plane surface, a fence-like member having the contour ofthe section of sound reflecting areas in said room on a small scale, mounted on said plane surface, and a source of light positioned at a point corresponding'r to the point of sound emanation in said room. n

3. An apparatus for photographing the paths and intensity of sound waves from a given source, in a room', comprising a plane surface on which is mounted a miniature fence-like member having the contour of a cross-section of said room, in combination with a source of light positioned at a point corresponding to the point of sound emanationI in said room, and means to record the light intensities on said plane surface.

4. An apparatus for picturing the paths and intensity of sound Waves from a given source in a room, comprising in combination a sensitized surface, a miniature light-reflecting fence-like memberhaving the contour of a cross-section of said room adapted to be mounted on said surface, and a source of light positioned at a point corresponding to the point of sound emanation in said room.

An apparatus for picturing the paths and intensity of sound Waves from a given source in a room, comprising in combination,

.a sheet of sensitized paper, a regulable reflecting metal fence-like miniature member having they contour of a cross-section of said room, and an incandescent bulb positioned at f a` point corresponding to the point of sound l emanation in said room.

6,. An apparatus for picturing the paths and intensity of sound waves from a given source in a room, comprisingl a plane surface on which is mounted a: miniature fence-like member having the contour of a cross-section `of said room, incombination with a source of light positioned at a point corre- Y' Sponding to the point of sound emanation in said room, said contour surface in light-refleeting properties simulating the sound-absorbing properties of the corresponding surfaces of said room.

sitized paper on which is mounted a polished metal reflecting-strip providing a miniature member having the contour of a cross-section of said room, in combination with an electric incandescent bulb positioned at a point corresponding to the point of sound emanation in saidv room, said reflecting strip surface having been treated as to its light-reflecting surface to simulate the sound-absorbing roperties of the corresponding surfaces o said room. 'I

8. In apparatus for picturing the paths and intensity of sound waves from a given source in a room, a miniature light-reflecting contour wall representing a rtion of a cross-section of said room and o ructions to light positioned therein to throw shadows and f indicate the reflections of light from said wall.

v9. In apparatus forA picturing the paths and intensity of sound waves from a given source in a room, a-reflecting strip providing a miniature fence-like member havin 'the contour of a portion of a cross-section o said room, said reflecting-strip surface having been treated onv its light-reflecting surface to simulate the sound-absorbing properties of the corresponding surfaces of said room.

' 10. In apparatus for picturing the paths and intensity of sound Waves from a given` flecting surfaces-of the auditorium being represented in the miniature section by light-refleeting surfaces.

12. Means for studying the acoustic properties of an auditorium, which comprises in combination a miniature slice-section of the auditorium, a source of light as a representation of a source of sound, the sound-reflecting surfaces of the auditorium being represented in the miniature section by 'light-refleeting surfaces, and means for recording the relative intensities of light throughout the area of the section.

13. Means .-for studyingl the acoustic properties of an auditorium, which Vcomprises incombination a sheet of light-sensitive material, a miniature slice-section of the auditorium placed upon said sheet, and a source of light as a representation of a source of sound, the sound-reflecting `surfaces of the auditorium Walls being represented in the miniature by light-reflecting surfaces. v

14. Means for studying the acoustic .properties of an auditorium, which comprises in iio combination a sheet or' light-sensitive aterial, a. miniature slice-section of the au to` rum placed upon said sheet, and light-ob.- struting pins at particular points inthe miniature to. indicate by their shadows the direction of light.f I v In testimony whereof, I have subscribed my name.

. RALPH FORBUSH NoRRIs. 

